The classical chemical analysis method for the determination of the degree of neutralization of a known concentration of an acid in solution is to titrate the solution with a base. The endpoint of the classical titration is progressively less distinct as the pK.sub.a of the acid is larger. If the pK.sub.a of the acid is larger than about 8, then the classical titration method becomes difficult. D. Skoog & D. West, Fundamentals of Analytical Chemistry, Chapter 14 (1963). The pK.sub.a of phenol is about 10.
Membranes have been widely used in chemical analysis methods. Cortes and Davis, U.S. Pat. No. 4,529,521, used membranes to determine components of interest in latex serum. Stevens, Jewett and Bredeweg, U.S. Pat. No. 4,751,004 used membranes to suppress an Ion Chromatography eluent. Morabito, Melcher, Hiller and McCabe, U.S. Pat. No. 4,962,042, used membranes in a Gas Chromatography system. Stevens, Frawley, Swart, Harris, Diedering, Nicholson and Rothman, U.S. Pat. No. 4,837,161, used membranes to add reagent to a Flow Injection Analysis carrier stream. Melcher and Burt, U.S. Pat. No. 4,913,821, used membranes in a phenol analyzer. Melcher and Cortes, U.S. Pat. No. 4,775,476, used membranes in a Liquid Chromatography system. It is known that the protonated form of an organic acid will permeate across a silicone rubber membrane at a faster rate than the ionized form of the acid.